This invention relates to an IC tester using an electron beam, adapted for testing semiconductor integrated circuits.
In the conventional failure analysis of integrated circuits (ICs) and large scale integrated circuits (LSI), a thin probe connected to an oscilloscope is placed on a desired position of an IC for waveform observation. However, this method involves technical difficulties in testing specimens with high integration density and high operating speed. This is because the space resolution cannot be enhanced over the size of the probe, and the time response is restricted due to the capacitance and inductance of the probe.
As a solution to these problems, an IC tester using an electron beam (hereinafter referred to as EB tester) has been recently developed. The EB tester has a space resolution of submicrons possessed by the electron beam and a time resolution of picoseconds. Accordingly, these testers can perform the failure analysis based on waveform observation as well as the detection of a disconnection in the wiring in LSI's.
FIG. 1 is a schematic illustrating the principle of the EB tester which is described in an article entitled "Fault-Diagnosis & Failure-Analysis Techniques for IC Based on Stroboscopic SEM Method" in a Japanese technical journal, Electronic Materials, vol. 2, 1981. In FIG. 1, reference numeral 1 designates an electron optical column which comprises an electron gun 2, electron lenses 3 and 4, and a scanning coil 5. There are also shown a specimen chamber 6, an exhaust pump 7, a specimen (IC) 8, a scintillation counter 9, an electron-beam accelerating power supply 10, an electron-lens power supply 11, a scanning-coil power supply 12, a signal amplifier 13, a cathode-ray tubes 14 and 15, and a camera 16.
In this EB tester, an electron beam having a diameter of 0.01 to 0.1 microns is radiated to IC 8, and the response of secondary electrons thus emitted is indicated on cathode-ray tubes 14 and 15 to perform the fault diagnosis and failure analysis. More specifically, the failure analysis is achieved by detecting the secondary electrons from IC 8 by means of scintillation counter 9 with probes 18 of a probe card 17 kept in contact with input and output terminals of IC 8, as shown in FIG. 3. In FIG. 3, reference numerals 20 and 21 designate focusing lenses and an electron collector, respectively.
As shown in FIG. 4, the probe card 17 is formed of an insulating plate 22, which has a through hole 23 in the center for the passage of electron beams, probes 18 arranged on the periphery of hole 23, and electrical terminals 24 connected to probes 18.
Referring now to FIG. 2, there will be described the structure of specimen chamber 6. A specimen 8 such as a wafer is set on a specimen table 19. The specimen table 19 consists of a .theta. table 25 rotating in a plane perpendicular to the direction of electron beam irradiation, and upper and lower X-Y tables 26 and 27 each moving in X- and Y-directions in a plane perpendicular to the electron-beam irradiating direction, whereby the wafer 8 is positioned relatively to the electron beam. Also, the specimen table 19 is provided with a Z table 28 which moves in the direction of electron beam irradiation, i.e., in Z-direction. The Z table 28 is fitted with a connector 29 into which are plugged the electrical terminals 24 of probe card 17. After replacement of specimen 8, the probes are brought into contact with the wafer by the movement of Z table 29 in the Z-direction. The aforesaid tables are driven by motors (not shown) disposed in specimen chamber 6.
In replacing the specimen in this prior art EB tester, the probe card must be disengaged from the connector after removing the electron optical column, so that the downtime of the tester is extended. The probe card must be frequently replaced according to the type and size of specimens and the kind of tests, and must have easy operability because the specimen is tested in vacuum. Since the probe card is inserted into the connector, the probe-card terminals are rubbed against the connector every time the probe card is replaced. Accordingly, the electrical contact is made unreliable by abrasion, with the result that the probe card and the connector cannot be used over a long period of time.
The specimens to be tested include wafers having integrated circuits formed thereon and packaged IC's. The above-mentioned probe card cannot be used for testing the packaged IC's. Therefore, a special holder is required for testing packaged IC's. In this case, the signal cable for the probe card are not available, so that an exclusive signal cable from the holder must be led outside of the specimen chamber. The signal cable has usually a coating of fluorine-containing polymer, which will produce a noticeable amount of gas. The addition of the signal cable for the packaged IC requires greater vacuum pump capacity, resulting in an increase in cost. If the tables move trailing these signal cables, the table drive motors will have load fluctuations, and the signal cables will be reduced in reliability.
Since the table drive motors are located in the specimen chamber, the movement of the tables will cause magnetic fluctuations in the specimen chamber unless the motors are perfectly magnetically shielded. It is difficult, however, to provide perfect magnetic shielding, so that the magnetic fluctuations cannot be avoided. Therefore, the electron beam is influenced by the magnetic fluctuations, and the space resolution, which constitutes an advantage of the stroboscopic SEM method, deteriorates.